Conventional atomic emission spectrometry, where samples to be analyzed are usually liquids or solids, has had to deal with spectral interferences that most often arose from light emitted by other metals present at high concentrations in the sample. The spectral interference most often results in an erroneous analysis for the element of interest due to spectral contributions from the interfering element at the detection wavelength of the element of interest. If the source (element) of the interfering emission can be identified, certain steps can be taken to account for the interference and minimize or eliminate any attendant effects.
However, continuous emissions monitoring for hazardous air pollutant metals is an emerging technology. Prior to this time, there has been no documentation reporting a satisfactory solution to correcting the problems associated with this monitoring. This is primarily because performance limitations of existing instrumentation did not permit detection of airborne metals at sufficiently low concentrations where molecular gas emissions would be considered a problem. The goals of developing a continuous emissions monitor for airborne metals having low sensitivity and accurate measurements at these low levels have not been met heretofore. One approach used by the applicant was to apply post measurement calculations in an attempt to correct for the type of molecular gas spectral interferences described above. This approach is neither accurate nor does it support real-time measurement. No other prototype technology for continuous monitoring of airborne metal emissions is sufficiently sensitive so that the issue of molecular gas spectral interference must be contended with.
Several prototype technologies have been attempted in recent years to address the need for instrumentation and methodology capable of continuous, real-time measurement of hazardous air pollutant (HAP) metals in the flue gases of large scale combustors such as waste incinerators, cement kilns, and coal-and-oil-fired power plants. Among these, however, a noteworthy continuous emissions monitor (CEM) has evolved that is based on the introduction of sample stream air into an argon inductively coupled plasma (ICP) and has been most successful in fulfilling the requirements of this demanding application. This CEM is described in U.S. Pat. No. 5,596,405 and monitors hazardous air pollutant metals. In summary, the patent describes a sample air stream that is extracted from an incinerator stack or other duct to be monitored, and introduced into an ICP spectrometer via a device henceforth referred to as a sampling interface. The ICP in this case serves as the actual metal analyzer and is capable of differentiating between various metal pollutants and determining directly, their concentration in the sample air stream. In the plasma, metal atoms are vaporized and excited. This excitation could take place in any number of sources including, but not limited to, inductively coupled argon plasmas, inductively coupled plasma sustained on gases other than argon, microwave-induced plasmas, electrical spark plasmas, arc-induced plasmas, laser-induced plasmas and analytical combustion flames. Once the metal atoms are vaporized and excited by one of the above sources the emission of characteristic wavelengths of light results. A suitable spectrometer is used to differentiate between the various light wavelengths while at the same time, discriminating against background emission of light from the plasma itself. Most metal elements emit light at several wavelengths simultaneously due to the multiplicity of atomic energy levels associated with a given element. The patent adopts a customary practice and selects the most intense emission wavelength for each element for the purpose of obtaining maximum measurement sensitivity. The patented method involves continuous extraction of sample air from a smokestack under strictly isokinetic conditions and frequent, periodic injection of aliquots of that sample air directly into the argon plasma using a plasma torch specifically designed for this application. As presently configured, the CEM of the patent is capable of sensitive, simultaneous analysis of all of the HAP metals at 1-2 minute intervals. Detection limits less than 1 microgram per dry standard cubic meter (DSCM) are routinely achieved for many of the HAP metals. Detection limits are achieved for all HAP metals, well below present or proposed regulatory compliance emission limits for incinerators and cement kilns, see "Revised Standards for Hazardous Waste Combustors, Proposed Rules," Federal Register, Apr. 19, 1996, Vol. 61, No. 77, p 17357-17536. Because, any monitors must comply to these regulations for this technology, a particularly high value is placed on measurement accuracy. Spectral interferences can easily compromise accuracy in any spectrochemical technique, and this, unfortunately, was found to be especially true in the patented monitor. The reason for the compromise in accuracy was primarily due to fact that the flue gases of furnaces and incinerators typically contain elevated levels of water vapor, molecular species, such as CO.sub.2, CO, NO.sub.x, and other products of incomplete combustion in addition to metallic and organic pollutants. Introduction of these molecular species into the argon plasma was an unavoidable consequence of air sampling in the patented monitor. In addition, introduction of molecular species into an inductively coupled plasma often resulted in complex thermodynamic perturbation of the plasma, see Trassy, C. C., Diemiaszonek, R. C., J. Anal. Atomic Spectrosc., 9, 661 (1995). During the development and initial field evaluation of the patented CEM, the effects of molecular species on the argon plasma, including spectral interferences and suppression of atomic excitation, have been observed.
Thus, in accordance with this inventive concept, a need has been recognized in the state of the art for a method of monitoring air borne atomic metals by which spectral interferences due to molecular species are corrected to result in accurate measurement of the affected metals.